1. Field of the Invention
This invention broadly relates to a process for producing formaldehyde from a gas stream containing a mixture of hydrogen, hydrogen sulfide (H.sub.2 S) and a carbon oxide, wherein the carbon oxide is selected from carbon monoxide (CO), carbon dioxide (CO.sub.2) and mixtures thereof More particularly, this invention provides a method wherein hydrogen, a carbon oxide and hydrogen sulfide are first passed in contact with a catalyst comprising a porous alumina supported sulfided metal selected from the group consisting of molybdenum (Mo), chromium (Cr), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), zinc (Zn), cobalt (Co), copper (Cu), tin (Sn), vanadium (V) and mixtures thereof, optionally promoted by an alkali metal sulfide, to convert said hydrogen, carbon oxide and hydrogen sulfide to methyl mercaptans, (primarily methanethiol (CH.sub.3 SH)), and the methyl mercaptans are then passed in contact with a catalyst comprising certain supported metal oxides or certain bulk metal oxides in the presence of an oxidizing agent and for a time sufficient to convert at least a portion of the methyl mercaptans to formaldehyde (CH.sub.2 O) and sulfur dioxide (SO.sub.2).
2. Description of Related Art
Olin et al., U.S. Pat. No. 3,070,632 describes a catalytic process for producing methanethiol (CH.sub.3 SH) from a gaseous feed comprising a mixture of hydrogen, carbon monoxide (CO) and hydrogen sulfide (H.sub.2 S). Gases containing H.sub.2 S are often considered an unwanted waste stream. According to the patent, the gaseous mixture (preferably containing a stoichiometric excess of hydrogen and hydrogen sulfide) is contacted, at a temperature of at least about 100.degree. to 400.degree. C. and at a super-atmospheric pressure, with a sulfactive catalyst comprising a metal sulfide of a metal selected from the group consisting of molybdenum (Mo), chromium (Cr), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), zinc (Zn), cobalt (Co), copper (Cu), tin (Sn), vanadium (V) and mixtures thereof. The gas and catalyst are contacted at a space velocity from about 150 to 1500 liters of gas at standard temperature and pressure per liter of catalyst per hour. Promoters such as organic amines, may optionally be used.
Buchholz, U.S. Pat. No. 4,410,731; Haines, U.S. Pat. No. 4,449,006 and 4,536,492 and Barrault et al., Applied Catalysis, 33:309-330 (1987) also describe catalytic processes for producing methanethiol (CH.sub.3 SH) from a mixture of hydrogen, carbon monoxide (CO) and hydrogen sulfide (H.sub.2 S) (or elemental sulfur). According to these patents and the article, the mixture is contacted, at an elevated temperature and pressure, with a porous alumina supported sulfactive catalyst comprising a mixture of a sulfided metal selected from the group consisting of molybdenum (Mo), chromium (Cr), manganese (Mn), nickel (Ni), iron (Fe), zinc (Zn), cobalt (Co), tungsten (W), vanadium (V) and mixtures thereof, and an alkali metal sulfide. Boulinguiez et al., U.S. Pat. No. 4,665,242, describes a similar catalytic process characterized by the added step of removing water from the unreacted, recycle gases before returning them to the catalytic reactor.
The art has also identified methyl mercaptans, such as methanethiol (CH.sub.3 SH) and dimethyl sulfide (CH.sub.3 SCH.sub.3), as hazardous pollutants, and has suggested a variety of ways for their destruction. Noncatalytic gas phase oxidation of such reduced sulfur compounds has been shown to produce primarily sulfur oxide and carbon oxide products. A. Turk et al., Envir. Sci. Technol 23:1242-1245 (1989). Investigators have observed that oxidation of methanethiol (CH.sub.3 SH) and ethanethiol (CH.sub.3 CH.sub.2 SH) in the presence of single crystal metal surfaces (Mo, Ni, Fe, Cu) results in the formation of methane and ethane, nonselective decomposition to atomic carbon, gaseous hydrogen and the deposition of atomic sulfur on the metal surface via a stoichiometric reaction (See Wiegand et al., Surface Science, 279(1992): 105-112). Oxidation of higher mercaptans, e.g., propanethiol on oxygen-covered single crystal metal surfaces (Rh), produced acetone via a stoichiometric reaction at low selectivity and accompanied by sulfur deposition on the metal surface (See Bol et al., J. Am. Chem. Soc., 117(1995): 5351-5258). The deposition of sulfur on the metal surface obviously precludes continuous operation.
The art also has disclosed using catalysts comprising a two-dimensional metal oxide overlayer on titania and silica supports, e.g., vanadia on titania, for catalytically reducing NO.sub.x by ammonia to N.sub.2 and H.sub.2 O in the presence of sulfur oxides. Bosch et al., Catal. Today 2:369 et seq. (1988). Thus, such catalysts are known to be resistant to poisoning by sulfur oxides. It also is known that such catalysts, as well as certain bulk metal oxides catalysts, can be used to oxidize methanol to formaldehyde selectively. Busca et al, J. Phys. Chem. 91:5263 et seq. (1987).
Applicant recently made the discovery that a supported metal oxide catalyst can be used to oxidize methyl mercaptans, such as methanethiol (CH.sub.3 SH) and dimethyl sulfide (CH.sub.3 SCH.sub.3), selectively to formaldehyde in a continuous, heterogenous catalytic process without being poisoned by the reduced sulfur. On the basis of that discovery, applicant has envisioned the present process as a way of converting gaseous streams containing hydrogen, a carbon oxide and hydrogen sulfide to formaldehyde.